The E21 error code indicates that the console is not receiving the speed sensor signal that it is expecting. I provided the technical information for this treadmill in the images below. If you are confident in your technical abilities to safely check voltage on the unit, you can check for the 5 VDC pulsing signal when the belt is moved as explained in the technical data below. If the console is getting the proper signal then it will need to be replaced. If the console is not receiving the pulsing 5 VDC signal when the belt is moving then you will need to check the wiring and the speed sensor.
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website:
Sears Home Services